Whether listening to an MP3 player while traveling, or to a high-fidelity stereo system at home, consumers are increasingly choosing the in-ear earphone for their listening pleasure. This electro-acoustic transducer device has a relatively low profile that provides convenience for the wearer, while also providing very good sound quality. An in-the-ear headphone, also referred to as an “earbud”, has a receiver or driver (an earpiece speaker) inside a housing that has an acoustic output tube. The open end of the latter is to be inserted into the wearer's ear canal. The tube is a rigid member that may be fitted with a flexible and resilient tip or cap typically made of a rubber or silicone material. The tip may be custom molded for the discerning audiophile, or it may be a high volume manufactured piece. The tip has an inner diameter that is slightly smaller than the outer diameter of the output tube. The user stretches the tip outward (in a radial direction), to enable it to easily slide (in a longitudinal direction) over the open end of the housing's output tube. The tip is then released, which causes it to collapse inward and grip the output tube. When such a headphone is then is inserted into the user's ear, a flared portion of the ear tip becomes compressed against the ear canal wall and thereby creates a sealed (essentially airtight) cavity inside the canal. This provides the wearer with good acoustic isolation against external sounds.
Consumers can wear in-the-ear headphones while conducting various types of daily activity, including not just sitting calmly and listening to music, but also while walking, exercising vigorously, and talking on a telephone call. This increased level of physical activity, however, results in the lower frequency or bass sounds that are being heard by the user to be amplified inadvertently by the headphone. This may be due to the headphone tip forming a very good seal with the user's ear canal wall, thereby creating an occluded cavity therein. Studies have shown that the sound pressure levels in the ear canal, produced by an external vibration source, are greater in the occluded ear than in the un-occluded or open-to-free-air condition. This effect is also referred to as bone conduction, because the user experiences the effect of external vibrations through the skull or other parts of the user's body outside the ear canal.